1. Field of the Invention
The present invention relates to an image pickup unit for endoscope which consists of an objective optical system and a solid-state image pickup device, and has a maximum field angle of 150° or larger.
2. Description of the Prior Art
An electronic endoscope equipped with an image pickup unit which consists of an objective optical system and a solid-state image pickup device is used for observing a relatively narrow space such as an interior of a coelom.
For practical use of this electronic endoscope, an endoscope unit which has a relatively large field angle is used for facilitating to find out an abnormal matter such as a diseased portion.
In recent years, a medical endoscope, for example generally has a maximum field angle on the order of 90° to 140°. However, this medical endoscope requires a long time for observing the space such as an interior of the coelom without overlooking, thereby requiring a long inspection time.
In order to shorten this inspection time, the endoscope which has a larger field angle is required. A field angle of 150° or larger permits observing a wider range in a tubular coelom, therefore being preferable for shortening the inspection time.
Known as conventional examples of an objective optical system for endoscope which has a field angle of 150° or larger are described in literatures mentioned below:
Japanese Patent Kokai Publication No. Hei 3-200911
Japanese Patent Kokai Publication No. Hei 4-275514
Japanese Patent Kokai Publication No. Hei 10-39206
Japanese Patent Kokai Publication No. 2002 72085
Japanese Patent Kokai Publication No. 2002-328299
Out of these conventional examples, objective lens systems for endoscope having field angles of 150° or larger are described as seventh and eighth embodiments of Japanese Patent Kokai Publication No. Hei 3-200911 and a fifth embodiment of Japanese Patent Kokai Publication No. Hei 4-275514 respectively. However, these objective lens systems for endoscope have not been mass manufactured as commercial products.
Furthermore, lens systems which are not objective lens system for endoscope but have field angles of 150° and larger are described in the Japanese Patent Kokai Publication No. Hei 10-39206, Japanese Patent Kokai Publication No. 2002 72085 and Japanese Patent Kokai Publication No. 2002-328299. The lens systems described in these publications are lens systems which are to be used in still video cameras and monitor cameras.
An endoscope which has a maximum field angle on the order of 90° to 140° is problematic in that the endoscope requires complicated and a long time operations since the endoscope requires an operation to bend a tip of the endoscope for observing a reverse side of a fold, for example, when the endoscope is used for observing an object having a fold in a tubular coelom such as a colon.
The objective lens system described in the above-mentioned Japanese Patent Kokai Publication No. Hei 4-275514 is a lens system which consists of three lens components of four lens elements and is problematic in that the lens system can difficultly be manufactured stably since a change of a field angle of the lens system is remarkably influenced by manufacturing errors of the lens elements, manufacturing errors of a lens barrel and manufacturing errors at an assembling stage of an image pickup unit, the influences are remarkable in particular.
Furthermore, this objective lens system for endoscope is inconvenient for observation since the lens system remarkably distorts marginal portions of an image, thereby collapsing an image of an object at marginal portions.
On the other hand, the objective lens system described in the Japanese Patent Kokai Publication No. Hei 3-200911 produces little distortion and convenient for observing marginal portions of an image, but has too low a magnification at a center portion of the image. Therefore, this objective lens system does not permit observing an enlarged image of a portion which is to be observed in details. Furthermore, this objective lens system has a most object side surface which is remarkably protruding, whereby a lens element is easily damaged and can not drain water well.
Furthermore, each of the lens systems described in the Japanese Patent Kokai Publication No. Hei 10-39206 and Japanese Patent Kokai Publication No. 2002 72085 uses a most object side lens element which has an outside diameter remarkably larger than those of other lens elements. Therefore, an endoscope will have a form, for example, such as that shown in FIG. 1 or FIG. 2 when the lens system described in either of the publications is used an objective lens system for endoscope.
As apparent from these drawings, the most object side lens element has an outside diameter which is larger than that of an image pickup device as seen in a diametrical direction of an image pickup unit. When the lens system described in either of these conventional examples is used in an image pickup unit, the image pickup unit will be enlarged since a maximum outside diameter of an image pickup unit is determined by-an outside diameter of a most object side lens element.
Furthermore, aspherical surfaces are used in the lens systems described in Japanese Patent Kokai Publication No. Hei 3 200911, Japanese Patent Kokai Publication No. 2002-72085 and Japanese Patent Kokai Publication No. 2002-328299. These lens systems therefore require high precisions for the aspherical surfaces and will degrade image qualities if the aspherical surfaces are formed with low precisions. In addition, the aspherical surfaces which can hardly be worked with high precisions have low yields and require high manufacturing costs.